The present invention pertains to a process for suppressing the influence of roll eccentricities on the strip thickness of the rolled material in a roll stand.
Eccentricities that influence the quality of the strip to be rolled are often found in rolling stands due to unevenly machined backup rolls or inaccurate bearing alignment. These eccentricities are manifested in the strip with the rotational speed of the roll affected by the eccentricity, usually the backup roll, depending on the rigidity of the roll stand and the material to be rolled. The frequency spectrum of the eccentricities and their negative influence on the strip includes basically the fundamental frequencies of the upper and lower backup roll; although there are also higher harmonic frequencies, these only appear with reduced amplitudes. Due to the slightly different diameters and rotational speeds of the upper and lower backup rolls, the frequencies of these backup rolls may differ.
In a process described in European Patent B-0 170 016, the roll eccentricities of the upper and lower backup rolls are simulated through the sum of the output signals of two oscillators connected in a feedback loop, and supplied to a position or thickness control for the roll stand to suppress the influence of roll eccentricities on the exit thickness of the rolled material. The oscillators work by the monitor principle, where the frequencies of their output signals are set according to the measured rotational speed of the rolls; the amplitude and phase of the output signals are corrected according to the difference between the summed output signal of the two oscillators and another sum signal obtained from the measured rolling force multiplied by the sum of inverse values of the roll stand's and the rolled material's rigidity and the measured actual value of the roll screw-down. The oscillators can be implemented as digital filters connected to the other analog position or thickness control of the roll stand through analog/digital converters and digital/analog converters. Assuming that the dynamics of position control (i.e., the dynamics of the control circuits and actuators used for regulating the screw-down position of the rolls) are negligible, the process in this European patent provides proper compensation for roll eccentricity. The measurement of the rolling force and thus the compensation for roll eccentricity, however, can be influenced by friction in the roll stand.
In a process described in U.S. Pat. No. 4,648,257 for compensating for roll eccentricities, the thickness of the rolled material is measured after its exit from the roll stand and used, together with the measured instantaneous rotation angle of at least one roll, for the ongoing calculation of estimated values for thickness changes in the rolled material. These estimated values are corrected, on the basis of the measurement delay, resulting from the distance of the thickness measurement point from the roll gap (i.e., the point of thickness change of the rolled material) convened into the corresponding rotation angle of the roll. The corrected estimated values, referenced to the rotation angle, are then supplied to the position or thickness control to compensate for eccentricities. The exact determination of the instantaneous rotation angle of the rolls is, however, considered relatively difficult especially due to the rough environment around the roll stand.
An object of the present invention is to provide a process for compensating for roll eccentricities without the need for measuring the rolling force or the instantaneous rotation angle of the rolls.